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Biomass is a term used to describe any material of recent biological origin, including plant materials such as trees, grasses, agricultural crops, and even animal manure. The bulk composition (in terms of the amounts of cellulose, hemicellulose, and lignin) is, as might be expected variable opening upon the source and types of the biomass (Table B-17).

Table B-17 Bulk composition of different biomass type (% w/w, dry basis).

Biomass type	Cellulose	Hemicellulose	Lignin
Peat	10	32	44
Rice husks	30	25	12
Straw (wheat)	40	28	17
Wood (bark)	34	16	34
Wood (hard)	39	35	20
Wood (soft)	41	24	28

The elemental composition also shows some variation with the source of the biomass (Table B-18).

Table B-18 Elemental composition, ash production, and heat content of biomass types*.

Biomass, % w/w**	C	H	N	O	Ash
Miscanthus	49.5	6.2	0.6	43.7	3.3
Peat	53.1	5.5	1.3	38.1	5.6
Reed grass	49.4	6.3	1.6	42.7	8.8
Straw (wheat)	49.6	6.2	0.6	43.6	4.7
Sugar cane	49.5	6.2	0.5	43.8	3.7
Wood (bark)	47.2	5.6	0.3	46.9	3.9
Wood (birch)	48.8	6.0	0,5	44.2	0.5
Wood (pine)	49.3	6.0	0.5	44.2	0.5
*Listed alphabetical rather than by any preference. **Dry basis

The obvious data are those relating to the amount of oxygen in the biomass which will require (at some stage of the refining) a hydrodeoxygenation step.

In terms of the bulk composition, other biomass components, which are generally present in minor amounts, include (i) triglyceride derivatives, which are ester derivatives of glycerol three fatty acids, (ii) sterol derivatives, which are also known as steroid alcohol derivatives), (iii) alkaloid derivatives, which form a class of naturally occurring organic compounds that mostly contain basic nitrogen atoms, (iv) terpene derivatives, which are the primary constituents of the essential oils of many types of plants and flowers, (v) terpenoid derivatives, which are sometimes referred to as isoprenoids and form a large and diverse class of naturally occurring organic chemicals derived from terpenes – most are multicyclic structures with oxygen-containing functional groups), and (vi) wax derivatives, which are a diverse class of organic compounds that are lipophilic, malleable solids near ambient temperatures and include higher molecular weight alkane derivatives and lipids, typically with melting points above approximately 40°C (104°F) and melt to give low-viscosity liquids. Waxes are insoluble in water but soluble in organic, nonpolar solvents. Natural waxes of different types are produced by plants and animals and occur in crude oil).

This list (above) includes everything from primary sources of crops and residues harvested/collected directly from the land, to secondary sources such as sawmill residuals, to tertiary sources of post-consumer residuals that often end up in landfills. A fourth source, although not usually categorized as such, includes the gases that result from anaerobic digestion of animal manures or organic materials in landfills (Wright et al., 2006).

Thus, knowledge of the composition of a biomass feedstocks is critical to the selection of the varieties with optimized properties for downstream conversion (De Jong, 2014). This can be partially achieved by selecting varieties with biomass composition that are better suited to the conversion process. Lignocellulosic biomass displays considerable recalcitrance to biochemical conversion because of the inaccessibility of its polymer components to enzymatic digestion and the release or production of fermentation inhibitors during pretreatment. If the ratio of hemicellulose, cellulose, and lignin in a woody biomass feedstock was optimized for the specific biochemical conversion method, then the pretreatment methods could be reduced or avoided.

Plants use the light energy from the sun to convert water and carbon dioxide to sugar derivatives (photosynthesis) that can be stored within the plant system. Some plants, such as sugar cane and sugar beets, store energy as simple sugars, while other plants store the energy as more complex starch derivative. These plants include grains like corn which are prominent food sources. Another type of plant matter – cellulosic biomass – is composed of complex sugar polymers, and is not generally used as a food source. Cellulosic feedstocks under consideration for biofuels include (i) agricultural residues which is the leftover material from crops, such as the stalks, leaves, and husks of corn plants, (ii) forestry wastes, which include chips and sawdust from lumber mills, dead trees, and tree branches, (iii) municipal solid waste, which includes household garbage and paper products, (iv) food processing and other industrial wastes such as black liquor, which is a paper manufacturing by-product (Table B-19), and (v) energy crops, such as fast-growing trees and grasses) developed just for this purpose.

Table B-19 Composition of black liquor.

Element	% w/w
Carbon	35.7
Hydrogen	3.7
Nitrogen	≥0.1
Oxygen	35.8
Sulfur	4.4
Chlorine	0.3
Potassium	1.1
Sodium	19.0

The main components of these types of biomass are (i) cellulose which is the most common form of carbon in biomass, accounting for 40 to 60% w/w of the biomass, depending on the biomass source and which is a complex sugar polymer, or polysaccharide, made from the six-carbon sugar, glucose – the crystalline structure of cellulose renders it resistant to hydrolysis, the chemical reaction that releases simple, fermentable sugars from a polysaccharide; (ii) hemicellulose which is also a major source of carbon in biomass, at levels of between 20% and 40% w/w of the biomass and is a complex polysaccharide made from a variety of five-carbon and six-carbon sugar derivative – it is relatively easy to hydrolyze into simple sugars, but the sugars can be difficult to ferment, and (iii) lignin which is a complex polymer that makes up 10% to 24% w/w of the biomass and provides structural integrity in plants – it remains as residual material after the sugars in the biomass have been converted.

In the simplest sense, biomass is carbon based and is composed of a mixture of organic molecules containing hydrogen, usually including atoms of oxygen, often nitrogen and also small quantities of other atoms, including alkali, alkaline earth, and heavy metals. However, because of the wide variations in the character and properties of biomass, it is anticipated and realized that the character and properties of the biofuels produced from biomass are very dependent upon the initial biomass. The exception is the fuels produced by gasification and Fischer-Tropsch synthesis.

In addition to the chemical composition, three properties of biomass that are significant to the performance of biomass as a fuel are (i) mineral matter content, manifested in thermal processes as mineral ash, (ii) susceptibility of the mineral matter to slagging and fouling, and (iii) the volatile matter content. The mineral matter content is the mass fraction of biomass composed of non-combustible inorganic material. Grasses, bark, and field crop residues typically have much higher content of mineral matter than wood. Systems that are designed to combust wood can be overwhelmed by the volume of ash if other biofuels are used. Slagging and fouling are problems that occur if ash begins to melt during combustion, forming deposits on combustor surfaces (fouling) or leaving hard chunks of glassy material in the bottom of the combustion chamber (slag, often referred to as clinkers).

Certain mineral components in biomass fuels, primarily silica, potassium, and chlorine, can cause these problems to occur at lower temperatures than normal. Dirt contamination also adds to the mineral content and associated slagging and fouling problems, so it is important that biomass feedstock be as clean (dirt-free) as possible. Slagging and fouling is minimized by keeping combustion temperatures low. Alternately, some biomass combustion equipment is designed to encourage the formation of clinkers (often referred to in the singular form, clinker) but is able to dispose of the hardened ash in an effective manner.

The content of volatile constituents (or volatile matter) in a fuel is a lesser-known property that refers to the fraction of the fuel that will readily volatilize (turn to gas) when heated to a high temperature. Fuels with high volatiles content will tend to vaporize before combusting, whereas fuels with low volatiles will burn primarily char. This affects the performance of the combustion chamber and should be taken into account when designing a combustor.

Other properties such as the particle size and density of biomass fuels are also important as they affect the thermal processing characteristics (especially the combustion characteristics) of biomass, especially the rate of heating and drying during the thermal process. The feedstock particle size also dictates the type of handling equipment required. For example, the incorrect size fuel will negatively impact process efficiency and may cause jamming or damage to the handling equipment. Smaller-sized fuel is more common for commercial-scale systems because smaller fuel is easier to use in automatic feed systems and allows for finer control of the processing rate by controlling the rate at which fuel is added to the reaction chamber. Fuel particle size and density are probably the most overlooked factors affecting fuel performance and should be given careful consideration when selecting a fuel type. Bulk density is the mass of a material divided by the volume it occupies. Bulk density of granular materials is dependent on the manner in which it is handled insofar as freely settled material has a lower bulk density than tapped or compacted materials.